In Trials and Tribbleations, a DS9 episode, Worf is confronted with the sight of smooth foreheaded Klingons of the TOS era. Questioned by his peers he says "we do not speak of it", obviously embarrassed.

That was the admission that there are actual differences in TOS era Klingons and TNG onward, that we as the audience aren't supposed to just pretend otherwise like we do with styrofoam boulders and other TOSness.

The ENT arc gives us the explanation for what fans have wondered for years about